One type of solar collector includes a concentrator, such as a parabolic dish reflector, which forms a concentrated image of the sun onto the opening of a receiver. The concentrated solar radiation heats tubes within the receiver chamber that carry water or other working fluid. If the concentrated image of the sun strikes the receiver face plate due to a tracking failure, then the face plate may be burned. Similarly, if working fluid in the receiver tubes is lost or stops circulating, the tubes can be damaged from over-heating.
Devices have been suggested for closing the receiver opening, such as a cover that normally lies beside the receiver and which is slid or otherwise moved over the receiver opening. However, concentrated sunlight may damage the cover. It is possible to use a large cover plate and a mechanism for moving the cover plate far in front of the receiver opening, so that only diffused sunlight is encountered by the cover plate. However, mechanisms for storing such a cover plate out of the way and for moving it far in front of the receiver are cumbersome. A simple and inexpensive apparatus for blocking sunlight, with only diffused sunlight falling on the blocking apparatus, would be of considerable value in protecting solar concentrator systems.